There are two types of optical recording media, one of which is to form physically deformed portions such as holes or concavities at a certain place of a recording layer by irradiation with beam of energy, and the other of which is to form portions having changed optical properties such as refractive index and reflactance at a certain place of a recording layer by irradiation with a beam of energy.
Recording layers consisting essentially of low melting metal such as tellurium (Te) have heretofore been known as those used in either of the two types of the optical recording media (Japanese Patent L-O-P Publns. Nos. 71196/1983 and 9234/1983). Te coating, typical of low melting metallic coatings, is capable of forming thereon desired physically deformed portions or portions having been changed in optical properties (hereinafter generically called "bits") using a very low energy, and thus is very useful as a high sensitivity material. By sensitivity as used herein and hereinafter is meant that which is defined by the energy (mw/cm.sup.2) required for forming bits per unit surface area.
However, Te is oxidized with oxygen or moisture when allowed to stand in contact with the atmosphere, whereby it increases in transmittance and becomes transparent. Because of its thin coating as thin as about several hundred .ANG., the recording layer formed by using Te alone markedly decreases in sensitivity when it increases in transmittance by oxidation of Te contained in such a thin recording layer. That is, when the recording layer composed of Te alone is oxidized, it increases in melt temperature and evaporation temperature and, at the same time, it diminishes in absorption of energy such as light as it becomes transparent, with the result that a large energy is required for forming bits and thus the recording layer markedly decreases in sensitivity. For instance, when a Te coating formed on a substrate is allowed to stand in the circumstances of 70.degree. C. and 85% RH, its sensitivity decreases by about 20% in about 5 hours, and by about 50% in about 25 hours.
With the view of solving such problems as mentioned above, there are taken various measures to prevent oxidation of the Te coating. It is known as one of these measures that the Te coating is coated on the surface with a stable inorganic substance. Though this measure is effective in preventing the Te coating from its being oxidized, it has not been put to practical use, because the use of the stable inorganic substance to be coated on the Te coating results in decrease of sensitivity of the Te coating and is also expensive. On one hand, it is also known to coat a Te coating on the surface with plastics, but this measure is of little real use in preventing the Te coating from being oxidized, because the plastics permit oxygen or moisture to permeate therethrough with relative ease, though the plastics are advantageous in that because of their low thermal conductivity, they are low in the degree of marring the sensitivity of the Te coating.
Further, proposed is a technique intended to prevent a Te coating as a recording layer from being oxidized by virtue of incorporation into the Te coating of C and H (Japanese Patent Publn. No. 33320/1984).
This proposed technique provides insufficient oxidation resistance of the recording layer yet when the content of C is decreased, and that C/N at the time of reading the recorded information decreases when the content of C is increased in order to improve oxidation resistance.
With the view of solving such problems as referred to above, Japanese Patent L-O-P Publn. No. 63038/1984 discloses optical recording media having recording layers consisting essentially of Te and additionally containing Cr. As stated in this publication, it has been known that when Cr is contained in a recording layer consisting essentially of Te, oxidation resistance of the resulting recording layer is improved in proportion to the content of Cr in said recording layer, and thus duration of life of the optical recording medium having such recording layer as mentioned above may be prolonged.
Optical recording media having recording layers consisting essentially of Te and additionally containing Cr as disclosed in the above-cited publication, however, possess such a drawback that the recording sensitivity decreases when large amounts of Cr are contained in the recording layers. On that account, it has been a common practice from the standpoint of improvement in oxidation resistance and in recording sensitivity that the content of Cr to be contained in a recording layer consisting essentially of Te is decided so as to amount of 5-15% by weight based on the Te present in the recording layer, as indicated in the above-cited Japanese Patent L-O-P Publn. No. 63038/1984.
However, the present inventors have found that optical recording media having Te recording layers containing 5-15% by weight, based on the Te, of Cr are still low in recording sensitivity in comparison with an optical recording medium having a recording layer consisting Te alone. As a result of extensive researches conducted by the present inventors on such optical recording media as having recording layers consisting essentially of Te and additionally containing Cr, it has now been found that in optical recording media having recording layers consisting essentially of Te and additionally containing a specific amount of Cr along with C and H, oxidation resistance is increased, nevertheless recording sensitivity markedly improves in comparison with optical recording media having Te recording layers containing large amounts of Cr and, at the same time, the recording margin is increased. The present inventors have also found that when optical recording media having on the substrate thereof recording layers consisting essentially of Te and additionally containing Cr, C and H are subjected to heat treatment, their recording sensitivity is further improved and, moreover, their recording margin is enlarged.